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124 FllOGHOPPEll BLIGHT OF SUGA R-CANK. 



It had been noticed that both drought and root fungus, which affect 

 the plant through the root?, produce a shortening nf the internodes of the 

 cane without any particular narrowing, in fact such short uiternodes (e.y. 

 those produced during the dry season) are frequently almost as thici< 

 than the longer internodes of the same cane. In '■ blight" on the other 

 hand one of the most typical symptoms is the narrowing of the 

 diseased canes, particularly when growth has just been resumed aftei- 

 an attack. 



On 30th October, 1919, as the outcome of a discussion with Mr. Nowell 

 as to the causes of certain symptoms, we removed the greater part of th(- 

 leaf blades from three otherwise quite healthy cane stools free from 

 froghoppers. One stool was left to grow. The otheis were trimmed 

 again seven and ten days later. 



The experiment was to test if the narrowing of the canes was due lo 

 leaf injury, as opposed to the shortening produced lij- root failure. 



Plate XI shows typical canes from the stools at the end of January 

 1920. A. shows three canes from an untoufhed control plant. B. 

 shows three canes from the stool from which the leaves were removed 

 twice at an interval of seven days, and C. shows three from the stool 

 kept trimmed for ten days. The stool with the leaves removed once 

 was unfortunately later infested with borer and the results were 

 unreliable. 



The narrowing of the canes is conspicuous and conclusive. In 

 addition several other points of great interest were brought to light. 



(1) The average weight of the main shoots on the control plant was 

 3 lbs. 10 ozs. The average weights of the main shoots of the two trimmed 

 plants were 2 lbs. 2 ozs. and 1 lb. 15 ozs. respectively. The complete 

 removal of the leaves from the cane had resulted in a reduction three 

 months later of about 50 pev cent, of its possible weight. 



(2) The untrimmed plants had several young and healthy shoots of 

 various ages. In the trimmed plants the youngest shoots were killed 

 and the medium sized ones were very severely injured, as is found in 

 froghopper blight. 



(3) The root systems of the trimmed plants, although not directly 

 injured became poorer and much smaller than those of the untrimmed 

 plants. The stools were looser in the ground and more easily pulled 

 out, exactly as is seen in typical blight. We thus have experimental 

 evidence that injury to the leaves can affect the root system in the 

 manner found in froghopper blight. 



(4) Several of the smaller shoots of the trimmed plants showed a 

 tendency to break across just above the upper nodes as is found in 

 typical cases of blight. 



In view of these experiments, which will be repeated more fully 

 this coming year, I think that the evidence is strongly in favour of the 

 view that the chief direct damage to the sugar cane is caused by the 

 destruction of leaf tissue following the sucking of the adult. 



Possible Infection or Toxin. 

 We have seen that tiie destruction of leaf tissue bj' the puncturing 

 of the leaves by the adult froghopper continues to increase for several 

 weeks after the original injury is inflicted. This is unusual and is 

 worthy of more complete investigation. There is no doubt that this 

 one fact makes the froghopper at least ten times more destructive than 

 it would otherwise be. 



It is a very strong indication that some form of poison or toxin is 

 introduced into the plant at the time of puncture. Kershaw (1913 H.> 

 describes the salivary glands of the froghopper and explains how the 



